Binders or binder systems for foundry cores and molds are well known. In the foundry art, cores or molds for making metal castings are normally prepared from a mixture of an aggregate material, such as sand, and a binding amount of a binder system. Organic and inorganic systems are currently used as binders in forming shapes from a mixture containing an aggregate material, such as sand. Typically, after the aggregate material and binder have been mixed, the resultant mixture is rammed, blown or otherwise formed to the desired shape or patterns, and then cured with the use of catalyst and/or heat to a solid, cured state.
In the foundry industry, the binder is typically from about 0.4 to about 6 percent by weight of the coated particle. Such binder coated foundry particulates have a particle size in the range of USA Standard Testing screen numbers from 16 to about 270 (i.e., a screen opening of 0.0469 inch to 0.0021 inch).
Typically, the particulate substrates for foundry use are granular refractory aggregate. Examples of refractory aggregates include silica sand, chromite sand, zircon sand, olivine sand, etc. and mixtures thereof. For purposes of the disclosure of the present invention such materials are referred to as "sand" or "foundry sand".
Regardless of the type of organic binder system, the organic binder used to produce desired shapes will volatilize during curing and/or bum out at metal pouring temperatures. Such processes produce smoke, odors and additional unwanted and harmful emissions which can result in the need to comply with applicable local and central government regulations. Another deficiency of some organic binder systems is their relatively short bench life. To obviate the deficiencies of the organic systems, some foundries use inorganic binder systems. One type of inorganic binder which is widely applied is an aqueous solution of a silicate, such as sodium silicate, i.e., water glass. (See U.S. Pat. No. 4,226,277 herein incorporated by reference). The solution usually contains 40-50% by weight of a sodium silicate having a weight ratio of SiO.sub.2 :Na.sub.2 O from 2.0:1 to 3.2:1.
U.S. Pat. No. 4,504,314, herein incorporated by reference, discloses mixing alkali metal silicate, glycosylated polyhydric alcohols, and optionally an oxyanion salt, with sand, and the resultant mixture is formed into a mold or core. After production, carbon dioxide gas is then blown through the mold or core. Due to the chemical reaction between the sodium silicate and the carbon dioxide, a bounded mold or core is formed.
In another method, deemed the self-set silicate process (or the "no-bake" process) described by Highfield et al, "The Mechanism, Control and Application of Self-Setting Sodium Silicate Binder Systems", AFS Transactions (1982) Vol. 90, pp. 201-214 (herein incorporated by reference), curing or hardening of the silicate shape is accomplished by the addition of organic esters as catalysts in the particulate mixture.
U.S. Pat. No. 4,416,694, herein incorporated by reference, discloses a foundry sand composition which comprises particulate sand, aqueous sodium silicate as binder and alkylene carbonate as hardener.
U.S. Pat. No. 4,983,218, herein incorporated by reference, discloses that aqueous solutions of alkali metal silicate are hardened using blends of alkylene carbonates and aliphatic alcohols such as alkylene diols, polyalkylene glycols, or hydroxyalkyl ethers.
Although the binding properties of the silicates are generally satisfactory they, when compared to organic systems, exhibit lower flowability of the binder/aggregate mixture due to the high viscosity of the silicate and relatively high binder levels required for adequate strength. Additionally, when subjected to metal pouring or casting temperatures, the silicates tend to fuse making it difficult to remove the fused shapes from castings by mechanical shakeout methods. The fused shapes also lack water solubility which prevents their removal or dissolution by water dispersing.
A second inorganic system, comprised of an aqueous solution of a polyphosphate glass is disclosed in WO 92/06808 which is herein incorporated by reference. These binders, when cured, exhibit satisfactory strengths, excellent rehydration, and breakdown of the aggregate shape after being exposed to metal casting temperatures. Deficiencies of this binder system include: poor humidity resistance, softening of the aggregate system at high temperatures, which restricts its use in ferrous alloy applications; and when compared to the organic binders, low flowability of the aggregate due to relatively high binder levels required for adequate strengths.
A third inorganic system is known which is comprised of a major portion of a finely-divided refractory material mixed with a minor portion of a dry phosphate to which is subsequently added a minor portion of an aqueous alkali metal silicate as disclosed in U.S. Pat. No. 2,895,838 (the entire disclosure of which is incorporated by reference) to make gas curable molds. This composition is chemically reacted with a gaseous agent, such as carbon dioxide, to cure the composition by reacting the binder with an alkali metal carbonate formed on curing of the inorganic system with carbon dioxide.
Another known inorganic binder system, which includes a combination of silicate and polyphosphate, is disclosed in the work of D. M. Kukuj et al, "Modification of Waterglass with Phosphorus Containing Inorganic Polymers" (hereinafter "Kukuj et al", the entire disclosure of which is incorporated by reference). The method of preparing this binder involved processing of the silicate and polyphosphate at higher than ambient temperatures and pressures in an autoclave to cause a chemical reaction of the inorganic polymers. The binder was then coated onto sand and was cured using CO.sub.2 at ambient temperatures. By this work, only a low level of polyphosphate could be incorporated in the binder preparation. In addition, Kukuj et al found that the maximum strength system had only 5% polyphosphate modifier and the strength dropped off dramatically when the binder contained more than 7% polyphosphate. Kukuj et al also found that small additions of polyphosphate in their binder (about 1 to 3%) caused a dramatic increase in viscosity of the binder prior to its addition to aggregate. Thus, the deficiencies of this system include: high temperature and high pressure processing required to produce the binder; formation of new chemical compounds with high viscosity; and low flowability of the binder/aggregate system. Also, like U.S. Pat. No. 2,895,838, chemical interaction of the binder system with a carbon dioxide containing gas was necessary to cure the system.